If you have a stable 5.0 Volt supply just connect your stable 5.0 Volts to your Vin pin. There is no reason to take 5.0 Volts and use a boost converter to get it to 12 Volts only so the onboard regulator can regulate the 12 Volts down to 5.0 Volts which is where you started from. Unless I am missing something?
Ron